Rattle (?) with throttle closed

[C420sailor]

'99 5.0L AWD. Runs smooth as butter, drives and shifts great.

Since I bought it (~600mi ago), it's had what sounds like a gentle, quiet rattle while driving when you close the throttle. Silent until you let off the throttle, then you'll hear it as RPM slowly decays through ~1700-1400, then it's gone. At first, I thought it was a belt idler or tensioner bearing that was going south, but I replaced all and the noise persists. Lubricated the cam synchronizer, no change. Can not duplicate while stationary, only while driving. Sounds like it's coming from in front of me, not the transmission. As soon as you ever so slightly crack the throttle open, it stops.

The above makes me think it could be vacuum related (closed throttle, under load)? Braking doesn't make it better or worse, though.

No codes, no other issues. Thoughts?

 

[imp]

'99 5.0L AWD. Runs smooth as butter, drives and shifts great.

Since I bought it (~600mi ago), it's had what sounds like a gentle, quiet rattle while driving when you close the throttle. Silent until you let off the throttle, then you'll hear it as RPM slowly decays through ~1700-1400, then it's gone. At first, I thought it was a belt idler or tensioner bearing that was going south, but I replaced all and the noise persists. Lubricated the cam synchronizer, no change. Can not duplicate while stationary, only while driving. Sounds like it's coming from in front of me, not the transmission. As soon as you ever so slightly crack the throttle open, it stops.

The above makes me think it could be vacuum related (closed throttle, under load)? Braking doesn't make it better or worse, though.

No codes, no other issues. Thoughts?

@C420sailor

How bad is it? Sound risky, or just annoying? I'd go in and check for something loose in the inlet air ductwork, outside or in, have seen thongs wiggling about making noise under no-load conditions. Possibility of front CV joint wear making noise when not transmitting hardly any torque. MAYBE front wheel bearing, more unlikely, though. They increase in noise when negotiating a curve, as weight of vehicle is "thrown" towards the inside of the turn. Had em make NO NOISE in straight ahead.

 

[C420sailor]

Nah, doesn't sound bad. It's so quiet that I can't really confirm that it is in fact a rattle. It's really hard to tell, and hard to pinpoint the location. I'm leaning away from driveline (CV, etc) because it's only in that very narrow RPM range, although I see your point about load. Turning and braking don't affect it. Like I said, reminds me of an idler bearing starting to go...like a small pulley wobbling/rocking.

I may see if I can plumb up a vacuum gauge to the passenger compartment to see if the needle does a little dance when the rattle happens. If so, I know it's vacuum related. If not, I'll look elsewhere.

Edit: Had the intake tract opened up yesterday. Cleaned the TB and MAF. No defects or loose objects in there.

 

[94Eddie]

Check your heat shields. I can't tell you how many times I tracked a nagging rattle to one of them. Most times they only rattled when the "harmonics" were just right at a certain RPM or in very specific conditions like you are seeing.

 

[C420sailor]

Solved. CV joint on front driveshaft was completely blown out. Had the driveshaft shop weld a new unit from Spicer on and balance the shaft. Quiet and smooth as silk.